1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to an image forming apparatus including a sheet inserting device which inserts an insertion sheet between sheets to which images are formed at an image forming apparatus main body.
2. Description of the Related Art
Conventionally, with an image forming apparatus such as a copier and a printer which includes an image forming portion, the image forming is performed easier when the sheet conveying speed at the image forming portion is slower. Therefore, the sheet conveying speed is preferred to be slow in order to obtain stable images. In addition, the cost becomes low in the case that the sheet conveying speed is set to be slow.
Further, when the sheets are continuously separated and fed by a sheet separating/feeding portion, the intervals between the sheets need to be enlarged in order to reliably separate and feed the sheets one by one. For example, with a retard separating/feeding portion having a pickup roller and a retard roller, in order to manage a failure of the sheet feeding with a single pickup operation, there needs to be a predetermined interval between feeding a sheet and feeding the next sheet for retrial.
Further, with an air separating/feeding portion which performs separation/feed by adsorbing a sheet to a belt with air, a predetermined interval needs to be set for adsorbing the sheet to the belt. As described above, the sheet conveying speed is preferred to be slow in order to obtain stable images at low cost. In addition, in order to reliably separate and feed the sheets one by one continuously, the sheets need to be conveyed at predetermined intervals.
On the other hand, with a conventional image forming apparatus, there is a case that high productivity is required in addition to the image forming and the sheet separating/feeding reliably at low cost. In order to meet such requirement, there is an image forming apparatus which accelerates sheets after performing separation/feed in a sheet separation period for the stable separation/feed and decelerates the sheets just before the image forming portion.
By structuring as described above, the sheet conveying intervals at the image forming portion can be shortened and the sheet conveying speed can be slowed at the image forming portion. Therefore, stable image forming at low cost and high productivity can be achieved.
Further, a conventional image forming apparatus includes a post-processing device which performs a post-process to the sheets on which images are formed and the sheets are transferred to the post-processing device after images are formed. Then, the post-processing device performs a process of bookbinding etc. to bind a sheet bundle. With such post-processing device, the sheet conveying speed is also slowed to synchronize to the image forming speed (the sheet conveying speed) of the image forming apparatus in order to perform the process reliably at low cost.
Furthermore, a sheet inserting device is arranged between a conventional image forming apparatus and a post-processing device. The sheet inserting device inserts a sheet which does not pass through the image forming apparatus or a sheet on which an image is previously formed (hereinafter, collectively called the insertion sheet) between the sheets which are sequentially discharged from the image forming apparatus.
In the case that the insertion sheet from the sheet inserting device is inserted to the sheet bundle as described above, the image forming apparatus forms the image so that the sheet is conveyed with spacing of the interval for the insertion sheet. The following is an example of when four sheets are continuously discharged from the image forming apparatus at time intervals of one second between the top ends of the sheets. When an insertion sheet is inserted between the third sheet and the fourth sheet, the image forming apparatus forms images at constant interval of one sheet per second from the first sheet until the third sheet.
However, the image forming timing is changed when an image is formed on the fourth sheet. Namely, the image forming is performed so that the time interval from the top end of the third sheet to the top end of the fourth sheet is to be two seconds. Then, when these four sheets pass through the sheet inserting device, one insertion sheet is inserted between the third sheet and the fourth sheet. Accordingly, when the five sheets are discharged from the sheet inserting device, namely, when conveyed to the post-processing device, the five sheets are discharged in five seconds at time intervals of one second.
Here, when inserting the insertion sheet at the sheet inserting device, the speed of the insertion sheet needs to be approximately equal to that of the sheet conveyed from the image forming apparatus. However, as described above, the sheets are conveyed from the image forming apparatus at low sheet conveying speed with narrow conveying intervals (inter-sheet distance). Therefore, when inserting the insertion sheets, the sheet inserting device inserts the insertion sheets at the low sheet conveying speed with the narrow conveying intervals.
Incidentally, even with the sheet inserting device, predetermined time is needed for stable separation/feed of the sheets, as described above. Therefore, the intervals of the insertion sheets become wide and the sheet conveying speed of the insertion sheets need to be set fast accordingly.
Consequently, when the insertion sheets are continuously inserted between the sheets conveyed from the image forming apparatus, the slow sheet conveying speed and the narrow inter-sheet distance which are the same as those of the sheets conveyed from the image forming apparatus cannot be achieved with the sheet inserting device. Therefore, with the conventional sheet inserting device, the sheet conveying speed of the sheets conveyed from the image forming apparatus is set to be fast or the inter-sheet distance is set to be wide (see Japanese Patent Application Laid-open No. 2001-26338).
With such a conventional image forming apparatus, when the sheet conveying speed at the sheet inserting device is set to be fast, the sheet conveying speed of discharging from the image forming apparatus main body differs from the sheet conveying speed of discharging from the sheet inserting device toward the post-processing device. Accordingly, the conveying speed of sheet receiving by the post-processing device has to be changed between the case that the post-processing device receives the discharged sheets directly from the image forming apparatus main body without disposing the sheet inserting device and the case that the post-processing device receives the discharged sheets from the disposed sheet inserting device.
Here, in the case that the sheets are discharged from the image forming apparatus main body directly to the post-processing device, a signal which indicates that the sheets are discharged is outputted from the image forming apparatus main body to the post-processing device. Further, in the case that the sheet inserting device is connected to the image forming apparatus main body, a signal which indicates that the sheet inserting device is connected is outputted from the image forming apparatus main body to the post-processing device.
Then, when the signal which indicates that the sheets are discharged from the image forming apparatus main body directly to the post-processing device is received or when the signal which indicates that the sheet inserting device is connected is received, the post-processing device first determines whether the sheet is from the image forming apparatus main body or from the sheet inserting device. Then, the post-processing device changes the sheet conveying speed of the transferring in accordance with the determination. As a result, the control becomes complicated and the transferring between the devices becomes unstable. Accordingly, there is a problem that conveyance jamming occurs.
On the other hand, with the conventional image forming apparatus, when the sheet conveyance intervals are widened at the sheet inserting device without changing the sheet conveying speed, the productivity per unit time is decreased due to the widening. Accordingly, there is a problem that the required high productivity cannot be achieved.
The present invention provides an image forming apparatus which can stably insert an insertion sheet without decreasing the productivity.